Avalanches

1. Avalanches Avalanches

2. Avalanches were first imagined as giant snowballs which increased in size from accretion of underlying snow

3. What are avalanches? • They are flows which move under the influence of gravity • They can be channelized or unconfined • In this sense, they are similar to pyroclastic flows, debris flows, etc.

4. Avalanches have severe consequences • Direct effects: • impact • burial • Indirect effects: • tsunamis generated if an avalanche enters a lake

5. Avalanche zones • a) Starting zone: where an avalanche is initiated • b) Avalanche track: where it goes • c) Runout area: where it dissipates

6. a) Starting zones of avalanches • Fs = Safety Factor • Fs = (shear strength)/(shear stress) • shear strength: internal resistance to movement • shear stress: force causing movement parallel to slope; increases with slope angle • If Fs is less than 1, then the slope is unstable and prone to failure

8. Layering of snow

9. Two weak layers within a slab

10. Initial failure - two types Failure at depth More dangerous Surface or near-surface

11. Loose snow failure

12. Slab failure

13. b) Internal structure of the flow Density and solids concentration gradient • 2 types of snow avalanche (a spectrum exists): • flow avalanches • airborne powder snow avalanches

14. Avalanche flow structure • Note the head, body, and tail of the flow • Vertical and lateral gradients in solids (i.e., snow) concentration and in density: • a lower dense portion which is highly hazardous and destructive • an overlying more dilute portion…also can be hazardous and destructive, since it is turbulent

15. Flow avalanches • Velocities up to 216 km/hr (60 m/s) • Flow heights 5-10 meters • Collisions of particles - granular flow • Initially tends to slide as a rigid body (similar to a landslide)… • …but rapidly breaks up into smaller particles and becomes a granular flow

16. Interior of the flow -There is a high-density core near the base of the flow -In this zone, particles collide, resulting in friction and producing heat -When the avalanche flow stops, freezing can occur, making the deposit very hard -sets like concrete High-density core

17. Mixed flow and powder avalanche

18. Airborne powder snow avalanches • Velocities can exceed 360 km/hr (100 m/s) • Flow thicknesses may exceed 100 meters • Essentially a highly dilute density current flowing down an incline: • partial entrainment of underlying snow by turbulent, erosive flow • dense core small or absent • powder avalanches may develop from flow avalanches, but the mechanisms are not well understood

19. Velocity Newly-fallen snow depth

20. Powder avalanche: note frontal zone of higher density, low-density cloud behind front

21. Fully-developed powder avalanche due to cascading down near-vertical cliffs

22. c) Runout area • Powder snow avalanches flow around obstacles, while flow avalanches do not • When powder snow avalanches hit a barrier, the lower dense portion of the flow is stopped, while the more dilute cloud behaves like a fluid which can flow around or over the obstacle

23. Some Canadian statistics • Activity1959-7474-89 • Fatalities: recreational 33 97 • activities • Fatalities: buildings, 53 6 • roads, worksites

24. Some interesting statistics from the Canadian Avalanche Association

25. Avalanche causes Causes of avalanches Trigger mechanisms

26. Types of snow and slopes prone to failure

27. Lee-side avalanche with cornice above

28. Survival

29. Some U.S. statistics Fatalities Property damage (thousands of dollars)

30. Mitigation • Avoid steep slopes, gullies • Close high-hazard areas to reduce risk and vulnerability • Set off explosive charges to artificially induce avalanches and remove the source material (unstable snow)

31. HAZARD MAPS, Alta, Utah: note the very fine line between zones of high hazard, potential hazard, and no hazard Note lack of vegetation, which could help dissipate avalanches

32. Engineering works • Reforestation: • to stabilize slopes and snow • Highways: • locate to avoid avalanche tracks • use of defense structures: deflectors, mounds, benches with dams

33. Avalanche avoidance

34. Use of defense structures

35. Starting zone defenses Terracing in avalanche starting zones • To help reduce avalanches from forming: • use of terraces • use of supporting structures

36. Supporting structures

37. Details of supporting structures

38. Some specific examples of mitigation attempts • Deflectors: • must be gradual, otherwise the avalanche will overflow the deflector

39. Arresters • Arresters are used to slow or stop avalanches • need adequate height; if too low, flow can accelerate above barrier, increasing damage

40. Splitters • These are placed directly in front of a single object • They redirect and divert the avalanche flow around the structure

41. Use of splitters on ski slopes

42. Mounds • These are used to retard flowing snow at the end of the runout zone

43. Detail of mounds

44. Snow sheds • These sheds allow the avalanche to pass over the structure

45. Note long dikes to prevent spreading Snow sheds

48. Current El Niño conditions

49. Global winter impacts from El Niño

50. Temperature anomalies from El Niño, Winter 1998